Firstly the Avid shorty and other similar cantis work fine. The problem is that people and tandem builders go wrong somewhere on the installation / setup. You need perfect cable routing.

Whatever you do, don't use that cable hanger pictured on the left. It is designed for the top hole to be held by the seat bolt and has 3 basic design faults.

1) They don't fit the seat bolt snugly, so twist sideways when you brake, losing power.
2) They are not angled and clearance for the brake outer is limited so it will alway have a nasty kink, again losing power.
3) Version you show has no adjustment. On some there is a screw for this which pushes against the seat tube, ruining your paint and moving the hanger out of the line of the brake, again losing power.

I substituted one of these for an integrated seat collar and brake cable hanger, solving in 20 minutes issues 1-3 and many hours spent trying to make my Trek Tandem's brakes work properly. Now I can lock the rear wheel while pulling a trailer using those Avid shorty cantis. It's not as good as a disc brake, but not bad either.